Wavelength division multiplexing (WDM) is a rapidly emerging technology that enables a very significant increase in the aggregate volume of data that can be transmitted over optical fibers. Traditionally, most optical fibers have been used to unidirectionally carry only a single data channel at one wavelength. The basic concept of WDM is to launch and retrieve multiple data channels in and out, respectively, from an optical fiber. Each data channel is transmitted at a unique wavelength, and the wavelengths are appropriately selected such that the channels do not interfere with each other, and the optical transmission losses of the fiber are low. Today, commercial WDM systems exist that allow transmission of 2 to 32 simultaneous channels.
WDM is a cost-effective method of increasing the volume of data (commonly termed bandwidth) transferred over optical fibers. Alternate competing technologies to increasing bandwidth include the burying of additional fiber optic cable or increasing the transmission speed on optical fiber. The burying of additional fiber optic cable costs on the order of $15,000 to $40,000 per Km. Increasing the optical transmission rate is increasing limited by speed and economy of the electronics surrounding the fiber optic system. One of the primary strategies to electronically increasing bandwidth has been to use time division multiplexing (TDM), which gangs or multiplexes multiple lower rate electronic data channels together into a single very high rate channel. This technology has for the past 20 years been very effective for increasing bandwidth; however, it is now increasingly difficult to improve transmission speeds, both from a technological and economical standpoint. WDM offers the potential of both an economical and technological solution to increasing bandwidth by using many parallel channels. WDM is complimentary to TDM, that is, WDM can allow many simultaneous high transmission rate TDM channels to be passed over a single optical fiber.
The use of WDM to increase bandwidth requires two basic devices that are conceptually symmetrical. The first device is a wavelength division multiplexer. This device takes multiple beams--each with discrete wavelengths and initially spatially separated in space--and provides a means of spatially combining all of the different wavelength beams into a single polychromatic beam suitable for launching into an optical fiber. The multiplexer may be a completely passive optical device or may include electronics that control or monitor the performance of the multiplexer. The input of the multiplexer is typically accomplished with optical fibers; however, laser diodes or other optical sources may be employed. The output of the multiplexer is typically an optical fiber.
Similarly, the second device for WDM is a wavelength division demultiplexer. This device is functionally the opposite of the multiplexer; it receives a polychromatic beam input from an optical fiber and provides a means of spatially separating the wavelengths. The output of the demultiplexer is typically interfaced to optical fibers or to photodetectors.
During the past 20 years, various types of WDMs have been proposed and demonstrated; see, e.g., (1) W. J. Tomlinson, Applied Optics, Vol. 16, No. 8, pp. 2180-2194 (August 1977); (2) A. C. Livanos et al, Applied Physics Letters, Vol. 30, No. 10, pp. 519-521 (May 15, 1977); (3) H. Ishio et al, Journal of Lightwave Technology, Vol. 2, No. 4, pp. 448-463 (August 1984); (4) H. Obara et al, Electronics Letters, Vol. 28, No. 13, pp. 1268-1270 (Jun. 18, 1992); (5) A. E. Willner et al, IEEE Photonics Technology Letters, Vol. 5, No. 7, pp. 838-841 (July 1993); and (6) Y. T. Huang et al, Optics Letters, Vol. 17, No. 22, pp. 1629-1631 (Nov. 15, 1992).
However, despite all of the above approaches, designs, and technologies, there remains a real need for a WDM device which possesses all the characteristics of: low cost, component integration, environment and thermal stability, low channel crosstalk, low channel signal loss, ease of interfacing, large number of channels, and narrow channel spacing.